System definition is an essential part of fault tree analysis. Usually, a diagram defining all functional interconnections and components of the system is used as the basis for the system definition. In addition to identifying component dependencies, the system definition must identify any important assumptions about the system and about conditions that indicate when the components are considered to have failed. Additionally, reliability parameters must be identified.

Because the top event sets the tone for the series of questions to consider when constructing the tree, it is important that the top event be clearly and concisely defined. For instance, if the definition for the top event is vague, the tree is likely to become too large and complex, resulting in a very unfocused analysis.

When you define the top event, it is often necessary to specify not only what the specific failure to analyze is but also to include a description of when this failure occurs. For example, perhaps the failure can occur only during a specific mission phase or portion of the mission.

To ensure meaningful analysis, you should perform the following steps when generating the system definition: